A conventional organic light emitting diode includes above a substrate an anode and a cathode with an organic functional layer system between the anode and cathode. The organic functional layer system may include one or a plurality of emitter layer(s) in which electromagnetic radiation is generated, one or a plurality of charge generating layer structure(s) each composed of two or more charge generating layers (CGL) for charge generation, and one or a plurality of electron blocking layer(s), also designated as hole transport layer(s) (HTL), and one or a plurality of hole blocking layer(s), also designated as electron transport layer(s) (ETL), in order to direct the current flow.
The limiting factor at present for the use of OLEDs in applications with a large temperature range is the operating life. It decreases greatly as a result of temperature increase, often for example by a factor of 3 upon an increase in the temperature of the OLED from 25° C. to 50° C. and by a further factor of 2 to 3 upon an increase from 50° C. to 75° C. The operating life in the case of a conventional OLED can be increased at a predefined ambient temperature by optimizing the organic functional layer system and the encapsulation. By way of example, the design of the OLED, for example the ratio of the marginal area to the luminous area, may be of importance for the intrinsic heating of the OLED. The thermal coupling of the OLED to other components may likewise lead to heat dissipation from the OLED, that is to say that part of the heat arising at the OLED may be dissipated. In a conventional method, the OLED is actively cooled by a Peltier element. However, the Peltier element is conventionally always an additional power consumer.
The current-voltage characteristic curve (I-U characteristic curve) of conventional OLEDs is greatly temperature-dependent. For the operation of an OLED with a predefined current density or luminance, a different voltage is required at different temperatures. In conventional OLEDs, the forward voltage thus varies greatly with the ambient temperature. Compared with operation at room temperature or higher temperatures, the OLEDs at low temperatures require a relatively high voltage for the rated current. For current regulation in light emitting diodes in the automotive sector, clocked switch mode power supplies (SMPS) and linear regulators are used. For applications in a large temperature range, for example in automotive applications, with the use of linear regulators, therefore, the maximum voltage dropped across the OLED is kept available for the entire temperature range.
In the case of a circuit including a linear regulator 504, the maximum voltage UOLED dropped across the OLED 502 is kept available as DC voltage, illustrated in FIG. 5. The voltage Ureg (Ureg=Uin−UOLED) not required by the OLED 502 is dropped across the linear regulator 504. The linear regulator 504 acts as a regulated variable resistor. The input voltage of the linear regulator is conventionally designed to be high enough that current is still supplied to the OLED at low temperatures. However, if the temperature of the OLED is high, at the linear regulator a very high voltage difference between input and OLED is compensated for in the form of waste heat at the linear regulator. At high ambient temperatures of the OLED 502, a large amount of heat is thus generated at the linear regulator 504. That is to say that the voltage difference Ureg between the maximum power required by the OLED 502 and the power required at a specific temperature is dropped across the linear regulator 504 and is converted into waste heat. With the use of linear regulators 504, the input voltage Uin is always greater than the forward voltage UOLED required for the load current IOLED, for example OLED rated current. The difference Ureg between input voltage Uin and (O)LED voltage UOLED is dropped across the linear regulator and is converted into thermal power at the linear regulator 504 by the flowing rated current IOLED. In order to increase the efficiency, this voltage difference should be minimal. At a given input voltage Uin, the tolerances of the OLED voltage UOLED should keep within the narrowest possible limits. The tolerances of the OLED voltage UOLED may be temperature-dependent, dictated by aging and/or dictated by manufacturing, for example. This can lead to thermal problems on the circuit board of the linear regulator 504 or require a higher outlay in the heat dissipation from the linear regulator 504, for example a larger heat sink. On account of the large voltage range, the efficiency of the entire lighting device is primarily low at high temperatures.
Therefore, in driver development recourse is conventionally had to switch mode power supply technology, which is replete with disturbances and relatively expensive. In this case, a DC voltage above the required maximum voltage of the OLED is converted into an AC voltage by a switch with a specific clock frequency. The AC voltage is rectified again and applied to the OLED. The DC voltage for the OLED can be varied by the clock frequency and the switching times (switches open/closed). The voltage can be varied with low losses which leads to high efficiency. However, the voltage at the OLED is not a pure DC voltage, but rather has voltage and/or current fluctuations (ripple), which are undesirable, however, at the OLED. Moreover, a high-frequency electromagnetic radiation is generated by virtue of the conversion of the clocked DC voltage into the AC voltage. Said high-frequency electromagnetic radiation can interfere with other applications, and is therefore measured and subjected to interference suppression in a complex manner. Furthermore, a higher outlay for the OLED, the circuit and the design is required in the case of a clocked voltage regulator compared with a linear regulator.
The circuit including a linear regulator 504 is simple, cost-effective and immune to interference compared with the clocked voltage regulator and no interfering electromagnetic fields are generated. However, the linear regulator 504 has a lower efficiency than switch mode power supplies. However, the clocked switch mode power supply has a significantly greater electromagnetic radio-frequency interference (electromagnetic interference, EMI). In a manner governed by the application, the switch mode power supplies are generally damped or subjected to interference suppression in a manner that is complex in terms of development and costly.